I have attempted to copy over the forms directory with a local copy I have extracted. That does not seem to have helped.
This is a rather major drama for us, so although I would prefer to function with a cool head, I am not certain that a cool head prevails.
Logically, I can recreate nearly everything that was on the site, however, it would be rather handy if it could just be magically solved. I have made some customisations to Silverstripe (extra page types), nothing that would have caused this however.
Should I re-install a newer SIlverstripe version on top?
Yeah this is nothing to do with a hack. As the error message you pasted states, and as StuM rightfully points out, the script has run out of memory, thanks to an overly restrictive/low limit being set.